In recent years, in accordance the with wide use of personal computers, word processors and facsimile machines in offices and homes, printers based on various printing methods have been developed as information output devices for the above devices. Among these printers, an inkjet printer, capable of color printing, having advantages of low operation noise, high-quality printing on various types of printing media, and further, its small size, is optimal for personal use in offices and homes. Above all, a serial-scan type inkjet printing apparatus (hereinbelow, simply referred to as a “printing apparatus”), in which a printhead performs printing while it is reciprocate-scanned across a print medium, is widespread in the market since it can print a high-quality image at a low cost.
The inkjet printhead (hereinbelow, simply referred to as a “printhead”) performs printing by discharging ink droplets toward a print medium from fine holes (hereinbelow, referred to as “discharge orifices”) of nozzles. At this time, in the printhead, the ink in each nozzle is reduced by discharge, and ink from an ink chamber is newly supplied by capillarity.
However, in a case where ink discharge is not performed for a long time, in the discharge orifice, water and solvent are evaporated and coloring material is deposited, and the deposited coloring material forms a film over the discharge orifice, which disturbs normal ink discharge. Since kinetic energy, generated for ink droplet discharge by the printing operation of the printhead, is consumed in breaking the film, a sufficient ink-droplet discharge speed cannot be obtained, thus an ink droplet cannot be applied to a desired position on the print medium.
In the printing apparatus, to prevent poor printing due to evaporation of water and solvent in the ink, the printhead is moved to the outside of the printing medium within a period where a film is not sufficiently formed over the discharge orifice, and ink discharge is made there (hereinafter, this discharge is referred to as “preliminary discharge”), thus the discharge orifice is maintained in a normal ink discharge status upon printing.
Generally, in a serial-scan type printing apparatus, upon returning movement of the carriage holding the printhead, it is determined whether or not a predetermined or longer period has elapsed from previous preliminary discharge, and if it is determined that a predetermined or longer period has elapsed, the printhead is moved to the outside the print medium, and the preliminary discharge is performed.
However, in a case where the preliminary discharge operation is frequently performed, the printing speed (throughput) is reduced, and the ink consumption for other operation than actual printing increases the running cost.
To overcome the drawbacks, Japanese Published Unexamined Patent Application No. Sho 63-252748, for example, proposes, on one hand, counting the number of driven times of each of plural print elements at predetermined time intervals and performing preliminary discharge if the count value is less than a predetermined value and, on the other hand, if the count value is equal to/greater than the predetermined value, not performing the preliminary discharge.
However, although the above conventional control is effective as long as a time interval to assure normal printing is longer than the period of one scanning of the printhead, if the time interval is equal or nearly equal to the scanning period, the efficiency of the control is low. In the conventional art, in a case where the time interval to assure normal printing is equal or nearly equal to the period of one scanning, it is necessary to perform the preliminary discharge upon each scanning considering that printing is concentrated immediately after the start of print scanning. In other words, it is necessary to perform the preliminary discharge upon each print scanning regardless of the count value of the number of driven times of the print elements.
In this case, the conventional control is utterly wasteful and the preliminary discharge is inefficient.